Pen barrel



FIG. 1 is an upper front right perspective view of a pen barrel showing my new design;

FIG. 2 is a front elevation view thereof, with the rear elevation view being a mirror image of the front elevation view;

FIG. 3 is a right side elevation view thereof, with the left side elevation view being a mirror image of the right side elevation view;

FIG. 4 is an enlarged top plan view thereof; and,

FIG. 5 is an enlarged bottom plan view thereof.

The broken lines in FIG. 4 illustrate environmental structure and form no part of the claimed design. 

CLAIM The ornamental design for a pen barrel, as shown and described. 